On October 4, 2010, some astronomers announced that asteroid
21 Lutetia
may be covered by a 2,000-feet (600-meter) deep layer of
dusty debris generated by billions of years of meteorite
impacts
(Jonathan
Amos, BBC News, October 4, 2010).
This conclusion was derived from data and images gathered
by the
ESA's
Rosetta
orbiter which passed near the asteroid on July 10, 2010,
while on its way to delivering the
Philae
lander on comet
Churyumov-Gerasimenko
in 2014. Of uncertain composition with unusual
surface colors and without traces of metals but many craters,
Lutetia looks like a carbonaceous asteroid in some ways, but
its reddish color also resembles iron-rich "M-type" asteroids
that are thought to be fragments of the cores of much larger
differentiated objects. The astronomers anticipate that
spectroscopic analysis of infrared data collected by Rosetta
will help to identify some of Lutetia's surface minerals.

Lacking visible traces of metallic minerals
on its surface, relatively large
21
Lutetia may
be a type of carbonaceous asteroid
(more).

The 60-mile (100-kilometer) wide asteriod is much larger than the
nine asteroids and four comets that have been previously visited
by Human probes. Despite its relatively large size, however,
Lutetia does not have enough mass for gravity to shape it into
a sphere, unlike Ceres. Sensors and
other instruments on Rosetta and its
Philae
lander also looked for evidence of a highly tenuous
atmosphere and magnetic effects, studied the asteroid’s density
as well as its surface composition, and may have caught stray
dust grains floating in space near the asteroid for on-board
analysis (more discussion, images, and video from ESA
news
release; and
Astronomy
Picture of the Day).

Asteroids

Asteroids are primoridal objects left over from the formation of the
Solar System. While some have suggested that they are the remains of
a protoplanet that was destroyed in a massive collision long ago, the
prevailing view is that asteroids are leftover rocky matter that never
successfully coalesced into a planet. Most planetary astronomers still
believe that the planets of the Solar System formed from a nebula of
gas and dust and ices that coalesced into a dusty
disk around the developing
Sun. Within the disk, tiny dust grains (and
ices in the colder environs beginning around two AUs inside of
Jupiter's orbit) coagulated into larger and larger bodies called
planetesimals, many of which eventually accreted into planets
over a period as long as a 100 million years. However, beyond the
orbit of Mars, gravitational interference
from Jupiter's huge mass prevented protoplanetary bodies from
growing larger than about 1,000 km (620 miles), by sweeping many
into pulverizing collisions as well as out into the
Oort Cloud or beyond Sol's gravitational
reach altogether.

Most asteroids are rocky bodies that orbit the Sun between Mars and
Jupiter in a "Main Asteroid Belt" that is centered around 2.7 times
the Earth-Sun distance (astronomical unit or AU) from Sol. Two
"clouds" of icy asteroids 60° ahead and behind Jupiter (and at
or near Jupiter's orbital distance from the sun) are called
"Jupiter
Trojans"
(diagram),
while two similar objects in Mars orbit are called
"Martian
Trojans." Some asteroids have been found inside Earth's orbit
(including many Near
Earth Objects), while others -- including burnt out or
dormant comets, such as perturbed
Edgeworth-Kuiper Belt objects called
"Centaurs"
-- are located beyond Saturn's orbit. Indeed, many have orbits that
cross Earth's path (see
orbit diagram
of near-Earth
Asteroid 4179
Toutatis), and while small asteroidal fragments hit the Earth every
day as meteorites, bigger asteroids are surmised to have landed with
impacts that killed off a significant share of life on the planet in
times past. While most asteroids may be only the size of pebbles, 16
asteroids have a diameter of 240 km (150 miles) and
Ceres, the largest, has a diameter of about
about 914 km (568 miles). On August 24, 2006, the
International Astronomical Union
voted at the end of its 26th
General Assembly to establish a new class of substellar objects
in the Solar System called "dwarf planets", which may eventually
encompass the largest, relatively round asteroids such as
Ceres and perhaps eventually
Vesta, Pallas, and Hygeia. (See an
animation of Ceres's orbit
around the Sun, with a table of basic orbital and physical
characteristics.)

As dark as soot, carbonaceous asteroid
253Mathilde is
rich in carbon compounds and other
dark substances common in the outer region of
the Main Asteroid Belt
(more).
Mathilde is like a
loose rubble pile with the density of water.

It has been estimated that the total mass of the Main Asteroid Belt
may total less than 1/1000th of the mass of the
Earth. Indeed, if all asteroids down to the
size of meter- or yard-sized boulders or less were combined together,
the resulting object would measure less than 1,300 to 1,500
km (810 to 930 miles) across, which is less than one third to one half
the diameter of the Earth's Moon. The Main
Asteroid Belt is only a small remnant of the material that once resided
in the region between Mars and
Jupiter, but once may have contained
between two to 10 Earth masses of material
(Dan Durda, "Ask
Astro," Astronomy,
December 2000). However, T-Tauri-type Solar winds from a very young
Sun, gravitational perturbations from
Jupiter developing nearby, and dynamic interactions with other large
planetesimals and protoplanets during the first 100 million years, and
continuing collisional grinding over the following 4.5 billion years
after the formation of the planets, interfered with the formation of a
substantial, single planet and caused most of the mass to be lost to
the rest of the Solar System and interstellar space.

Within the first 100 million years of the
Sun's birth, protoplanets agglomerated
from a circum-Solar disk of dust and gas,
but the development of Jupiter nearby
interfered with the formation of a single,
substantial planet in the Main Asteroid Belt.

Based on the composition of meteorites found on the Earth, most
asteroids may be composed of three materials: mostly (92.8 percent)
silicates (stone); metals (5.7 percent) iron and nickel; and the rest
as a mix of the those materials and carbon-rich substances. Asteroids
located closer to Mars and Earth that exhibit the same spectra are
composed of rocky minerals ("stone") mixed with iron. In contrast,
asteroids located farther away from the Sun on the Jupiter side of the
Main Asteroid Belt are generally darker, redder, and more icy,
presumably because they were not as well heated by the Sun and so have
a composition more like the primordial, circum-Solar dust disk out of
which the outer planets accreted about 4.5 billion years ago. Thus,
the outer asteroids may more closely resemble the icy planetary bodies
of the Edgeworth-Kuiper Belt and
Oort Cloud.

The larger asteroids or protoplanets may have accumulated enough
internal heat to "differentiate," whereby denser metals settled
downwards (and perhaps even form metallic cores) and left lighter rocky
("stoney") residues in their outer layers. On some asteroids, internal
heat may also have formed metamorphosized rocks, and volcanoes may even
have erupted. Although no asteroid in the Main Belt grew big enough to
hold on to an atmosphere, minerals found in some meteorites suggest
that liquid water was often present.

The near-Earth asteroid
Eros
is made of material that has not been much altered since its accretion
from the circum-Solar dust disk 4.5 billion years ago, never subjected
to the melting and separation of its materials into distinct layers --
e.g., differentiated into metallic core, mantle, and crust -- as
happened with the four inner planets (Earth,
Mars, Venus, and
Mercury). With an uniform density like the
Earth's crust, it is probably a fractured chip off a larger body,
perhaps another asteroid.
Eros is
roughly the size of Manhattan, NYC, USA (see
APOD 2001
and APOD 2009).

As Jupiter accreted into a giant planet, its
gravitational pull began to disturb the orbits of the nearest
planetesimals so that collisions became more violent. As a result, the
larger, differentiated protoplanets tended to be shattered into smaller
asteroids, and so many asteroids are believed to have formed from
their rocky or metallic debris based on spectral analysis of meteorites
that landed on Earth. In contrast, the asteroids found farther out are
thought to be made from planetary bodies that are less well
differentiated, if at all, as well as having a much higher ice content.

Icy "main belt comets"
in the outer reaches of
the Main Asteroid Belt
may have brought much
of the water that formed
Earth's oceans
(more).

On March 22, 2006, astronomers (including
David Jewitt and
Henry Hsieh) announced
that the near circular orbits of some icy asteroids in the outer
reaches of the Main Asteroid Belt suggest that they are part of
a population of "main belt comets." Three objects have been
found with near circular, flat orbits in the main belt
occasionally stream volatile materials, producing an observable
tail for weeks and months at a time as their orbits bring them
closer to the Sun. The astronomers speculate that past impacts
on Earth from such inner comets may have been an important
source of the water now found in Earth's oceans (IFA
press
release and web site on
main-belt comets;
and
Jewitt et al, 2006).

Near-Earth Objects
(NEOs) are
asteroids and comets that have been nudged
by the gravitational pull of nearby planets into orbits that cause
them to enter the Earth's vicinity. In terms of orbital elements,
NEOs are asteroids and comets with a perihelion (q) distance less
than 1.3 times the Earth-Sun distance (AU). Near-Earth Comets (NECs)
are further restricted to include only short-period comets -- that
is, with an orbital period (P)less than 200 years.
However, the vast majority of NEOs are asteroids, called Near-Earth
Asteroids (NEAs). NEAs are subdivided into the Aten, Apollo, and Amor
groups (that are categorized by their perihelion distance (q), aphelion
distance (Q), and their semi-major axes (a) of orbit) and the
Potentially Hazardous Asteriods (PHAs). PHAs are objects that can some
day harm life on Earth because of their proximity and size -- Minimum
Orbit Intersection Distance (MOID) with the Earth is 0.05 AU or less
with an absolute magnitude (H) of 22.0 or brighter.

One of the largest known PHAs is Toutatis, an asteroid that is nearly
a mile (1.6 km) long. No other PHA larger than 0.6 miles (one km) has
been found that moves around the Sun in an orbit so
nearly
coplanar -- inclined less than half a degree -- with Earth's,
extending from just inside Earth's orbit to a point deep within the
Main Asteroid Belt between Mars and Jupiter. On September 29, 2004,
Toutatis will pass just four Lunar distances from Earth, which is
closer than any other known PHA over the next 30 years. (See
NASA
article for more information.)

Analysis of data collected by spacecraft sent to a sample of asteroids
since the 1990s and estimation of rotation speeds for a large number
of asteroids have led to a startling conclusion. As of late 2001, no
known asteroid larger than 200 meters (656 feet) across has been found
to rotate faster than once every 2.2 hours. As a result, planetary
astronomers infer that, over time, repeated collisions appear to have
reduced most asteroids larger than a couple hundred meters (or yards)
into "rubble piles" that are only loosely held by their weak gravity,
which fly apart if spun too fast. For example, it has been estimated
that asteroid Mathilde has a density close to that of water. Smaller
asteroids, however, should be solid bodies because the shattered
fragments of an asteroidal collision should easily escape their
feeble gravity and fly off into space instead of settling back on
their surface as regolith.

Try the Orbit Viewer, originally written by
Osamu Ajiki of AstroArts and
modified by Ron Baalke of NASA's
Jet Propulsion Laboratory, to see real-time
orbit animations of the
known asteroids, Edgeworth-Kuiper ice bodies, comets, and planets.

Other
Information

Information and images about proposals to mine asteroids
(and comets) as economical sources of raw materials for
last space construction projects can be found at
Sol
Station.

David Seal (a mission planner and engineer at NASA's Jet
Propulsion Laboratory at CalTech) has a web site that generates simulated images of the Sun,
planets, and major moons from different perspectives and at different times of the year. Try
his Solar System Simulator.